Generally, exhaust gas flowing out from an engine through an exhaust manifold is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. Then, noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification (DeNOx) catalyst is one type of such a catalytic converter and purifies nitrogen oxide (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbon (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced in the DeNOx catalyst through oxidation-reduction reaction with the reducing agents.
Recently, a lean NOx trap (LNT) together with a selective catalytic reduction (SCR) catalyst is used to meet enhanced exhaust regulations. The LNT catalyst absorbs the NOx contained in the exhaust gas when air/fuel ratio is lean, and releases the absorbed NOx and reduces the released nitrogen oxide and the nitrogen oxide contained in the exhaust gas when the air/fuel ratio is rich atmosphere.
If the LNT and the SCR catalyst are simultaneously used, however, the SCR catalyst may be coated on a diesel particulate filter due to a limited space. The SCR catalyst on a diesel particulate filter (SDPF) is adapted to trap particulate material (PM) in the exhaust gas and to remove NOx contained in the exhaust gas using a reducing agent. In addition, if the amount of the PM trapped in the SDPF is greater than or equal to a predetermined amount of the PM, a temperature of the exhaust gas is raised and burns and removes the PM trapped in the SDPF. This is called a regeneration of the SDPF.
When the temperature of the exhaust gas is raised above a regeneration temperature to remove the PM trapped in the SDPF, the reducing agent is oxidized (the NOx is reduced) to generate an oxidation heat. The oxidation heat raises the temperature of the SDPF excessively and the SDPF may be damaged.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.